Absorption methods using an aqueous amine solution are commonly used for removing acid compounds, notably CO2, H2S, COS, CS2, SO2 and mercaptans present in a gas. The gas is deacidized by contacting with the absorbent solution in an absorption column (absorber), then the absorbent solution is thermally regenerated in a regeneration column (regenerator). A gas depleted in acid compounds is then produced in the absorber and a gas rich in acid compounds leaves the regenerator. For example, document U.S. Pat. No. 6,852,144 describes a method of removing acid compounds from hydrocarbons. The method uses a water-N-methyldiethanolamine or water-triethanolamine absorbent solution containing a high proportion of a compound belonging to the following group: piperazine and/or methylpiperazine and/or morpholine.
One limitation of the absorbent solutions commonly used in deacidizing applications is their insufficient absorption selectivity for hydrogen sulfide (H2S) over carbon dioxide (CO2). In fact, in some natural gas deacidizing cases, selective H2S removal is sought by limiting to the maximum CO2 absorption. This constraint is particularly important for gases to be treated that already have a CO2 content less than or equal to the desired specification. A maximum H2S absorption capacity is then sought with a maximum H2S absorption selectivity over CO2. This selectivity allows to recover an acid gas at the regenerator outlet having the highest H2S concentration possible, which limits the size of the sulfur chain units downstream from the treatment and guarantees better operation. In some cases, an H2S enrichment unit is necessary for concentrating the acid gas in H2S. In this case, the most selective absorbent solution possible is also sought. Tail gas treatment units also require selective removal of the H2S that is sent upstream from the sulfur chain.
It is well known to the person skilled in the art that tertiary amines or secondary amines with severe steric hindrance have slower CO2 capture kinetics than primary amines or little-hindered secondary amines. On the other hand, tertiary amines or secondary amines with severe steric hindrance have instantaneous H2S capture kinetics, which allows selective H2S removal based on distinct kinetic performances.
In 1950, Frazier and Kohl (Ind. and Eng. Chem., 42, 2288) notably showed that the tertiary amine N-methyldiethanolamine (MDEA) has a high H2S absorption selectivity degree over CO2 due to the distinct kinetic performances of this amine on these two gases. However, there are cases where using MDEA does not allow the desired H2S absorption capacity to be reached and involves insufficient selectivity. Thus, using MDEA for treating gases with high CO2 and H2S partial pressures, as it is for example the case for some natural gases, is of limited interest. This is also the case when it is desired to reduce H2S contents at low partial pressures, for example when treating refinery tail gas or syngas.
U.S. Pat. No. 4,405,581 discloses the use of absorbent solutions based on hindered secondary amines for selective removal of H2S in the presence of CO2. For the same use, patent U.S. Pat. No. 4,405,582 discloses the use of absorbent solutions based on diaminoethers where at least one amine function is tertiary. U.S. Pat. No. 4,483,833 discloses the use of heterocyclic tertiary aminoalcohol and aminoetheralcohols for removing H2S from a gaseous mixture comprising H2S and CO2. All these patents describe improved performances in terms of selectivity and capacity in relation to N-methyldiethanolamine.
The inventors have discovered that adding some organic compounds, notably in very low proportions, to a formulation containing water and at least one hindered tertiary or secondary amine allows to control the absorption selectivity during selective H2S absorption over CO2 from a gaseous effluent comprising H2S and CO2. Said organic compound, by increasing the dynamic viscosity of the aqueous solution in a controlled manner, allows to improve the H2S absorption selectivity in relation to CO2. Such a compound is referred to as “viscosifying compound” in the present description.
The inventors have also discovered that hindered tertiary and secondary amines in general and tertiary diaminoethers in particular are not equivalent in terms of performances for use in absorbent formulations containing a viscosifying compound allowing controlled selective H2S absorption over CO2. Surprisingly, 1,2-bis(2-dimethylaminoethoxy)ethane formulations containing a viscosifying compound distinguish themselves by their improved H2S absorption selectivity over CO2 in comparison with methyldiethanolamine formulations or other hindered tertiary and secondary amines containing the same viscosifying compound. This comparison can be achieved for example by increasing by the same multiplicative factor the viscosity of the various amine solutions through viscosifier concentration adjustment, for example by adjusting in each formulation the proportion of viscosifying compound so as to increase the viscosity of each amine solution by 40%.